1. Field of the Invention
The invention relates to a photomask and a method for fabricating the same, and more particularly, to a phase shift mask for extreme ultraviolet lithography and a method for fabricating the same.
2. Brief Description of Related Technology
As the semiconductor fabrication process becomes complicated and the integration density of semiconductor devices increases, finer patterns are increasingly demanded. To overcome the limiting resolution of the lithography process for forming fine patterns, extensive research has been conducted on extreme ultraviolet lithography technology which uses 13.5 nanometers (nm) extreme ultraviolet (EUV) light, which is shorter than a KrF or ArF light source generally used in an exposure process.
A photomask for extreme ultraviolet lithography has an optical reflection structure, in which a mufti-layered reflection layer including Mo/Si layers is formed on a transparent substrate, and an absorber pattern is formed on the reflection layer in a pattern shape that will be transferred onto a wafer. When extreme ultraviolet light is irradiated onto the mask for extreme ultraviolet lithography, the extreme ultraviolet light is absorbed at the absorber pattern and is reflected at the surface of the reflection layer.
Meanwhile, extreme ultraviolet light is irradiated or reflected not at a vertical angle but at an inclined incident angle with respect to the surface of the mask, for example, in a range from approximately 5° to approximately 6°. At this time, a height difference between the absorber pattern and the surface of the reflection layer forms a shadow region in which extreme ultraviolet light is neither irradiated nor reflected. Therefore, light irradiated onto and reflected from the mask for extreme ultraviolet lithography is incident on the wafer while its incident direction is slightly changing. Consequently, a shadow effect is caused. That is, a critical dimension or position of a pattern formed on the wafer is changed. To suppress the shadow effect, the height difference of the region where extreme ultraviolet light is absorbed and reflected is reduced by forming the absorber layer thinly in the fabrication of the mask for extreme ultraviolet lithography.
FIG. 1 is a plan view illustrating a structure of a phase shift mask for extreme ultraviolet lithography.
Referring to FIG. 1, a main chip region 110 where main patterns are formed is disposed at a center portion of the mask 100, and a frame region 120 is disposed to surround the main chip region 110. The main chip region 110 corresponds to an exposure field region where a one-time exposure process is performed, and the frame region 120 is a light shielding region which is not used in substantially transferring patterns such as bar codes onto the wafer.
However, if an absorber pattern is formed thinly, the absorber pattern does not serve as the light shielding pattern, and partially transmits the incident extreme ultraviolet light. Consequently, multiple exposure regions are formed during a wafer exposure process. That is, if the exposure regions are overlapped during the wafer exposure process, the overlapped regions are exposed doubly as the absorber pattern serving as the light shielding pattern is thinner. As a result, an image contrast of the region which is exposed doubly or multiply is degraded, causing wafer pattern failure.